Electrophotographic process

ABSTRACT

IN A MULTI-IMAGE ELECTROPHOTOGRAPHIC PROCESS, WHERE PLURAL PHOTOCONDUCTIVE TONER IMAGES ARE FORMED, APPLYING A MATERIAL TO A TONER IMAGE WHICH RENDERS THE IMAGE CONDUCTIVE BEFORE THE FORMATION OF THE NEXT PHOTOCONDUCTIVE TONER IMAGE. THE MATERIAL IS NON-ADHESIVE TO THE TONER, THE INITIAL TONER IMAGE THUS DOES NOT SERVE AS AN INSULATING LAYER FOR SUBSEQUENT PHOTOCONDUCTIVE TONER. PARTICULAR APPLICATION IS FORMED WITH TONER COMPRISING A TRANSPARENT INSULATING CORE WITH A SURFACE PHOTOCONDUCTIVE LAYER.

Oct. 9, 1973 SATORU HONJO ELECTROPHOTOGRAPHIC PROCESS Filed Dec. 28, 1971 FIG. I

FIG.2

,FIG.3

FIG.4

FIG.5

US. Cl. 961.2

United States Patent 3,764,312 ELECTROPHOTOGRAPHIC PROCESS Satoru Honjo, Asaka, Japan, assignor t0 Fuji Photo Film Co., Ltd., Kanagawa, Japan Filed Dec. 28, 1971, Ser. No. 213,135 Claims priority, application Japan, Dec. 28, 1970,

46/ 120,543 Int. Cl. G03g 13/22 3 Claims ABSTRACT OF THE DISCLOSURE In a multi-image electrophotographic process, where plural photoconductive toner images are formed, applying a material to a toner image which renders the image conductive before the formation of the next photoconductive toner image. The material is non-adhesive to the toner. The initial toner image thus does not serve as an insulating layer for subsequent photoconductive toner. Particular application is formed with toner comprising a transparent insulating core with a surface photoconductive layer.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to electrophotography employing photoconductive toners, and more particularly to an improved method in electrophotography wherein image formation of more than two times can be realized on a single substrate.

(2) Description of the prior art An electrophotography method utilizing photoconductive toners has been disclosed in the Japanese patent publication 22,645/ 1963. The method uses a minute powder having photoconductivity uniformly sprinkled over a conductive substrate, and the powder at that time, or after the dispersion, is charged to have a desired polarity as shown in FIG. 1. In FIG. 1, reference 1 designates a substrate and 2 designates photoconductive toner electrically charged into a negative polarity. The distribution of the electric charges within the toner layer is indicated schematically because the details thereof are not clear. A light image is then focused on the surface of the substrate 1.

Electric charge on the toner in a region strongly irradiated by the light rays is neutralized due to the photoconductive nature of the toner, and the electrostatic attraction between the substrate 1 and the toner 2 is thereby lost (see FIG. 2 where light is shown by the arrows). The toner thus irradiated by light is then selectively removed by a suitable means (for instance, by an air stream jet) from the substrate, so that the image focused on the substrate develops (see FIG. 3 where toner 2 is shown selectively removed).

The image thus obtained in FIG. 3 may be transferred to another material or fixed on the substrate. In the case where the toner includes a thermoplastic resin, fixation can be attained by the use of heat or a solvent. When different colors are given to the toner, and a toner of a different color is again dispersed (sprinkled) on the substrate to be subjected to the recurrence of the above described processes, an image having two colors can be obtained. In a similar manner, it will be apparent that a multi-color image is also obtained when the above described procedure is repeated as required.

3,764,312 Patented Oct. 9, 1973 In the past, since photoconductive toners of a comparatively simple composition have been employed, the toner image after fixation still contains a considerable amount of photoconductive components, whereby the existence of the first image does not hinder the formation of a second image or subsequent images on the same substrate.

Recently, however, photoconductive toners of superior behavior have been developed, and these toners are constructed as disclosed, for instance, in Japanese patent publication No. 12,385/ 1969, Belgium Pat. 710,572, etc. They may be from a transparent core typically consisting of an insulating resin, the outer surface of which is coated by a thin layer of a photoconductive substance. Such toners exhibit a lower residual potential after irradiation with light, and are far superior in comparison with toners of a uniform construction (having the same composition throughout the toner).

Although the newly developed toners are superior to conventional ones as described above, such toners are converted into an insulating layer having no photoconductivity when they are fixed by application of a solvent or heat and their useful structure is thereby destroyed. To be more specific, when the image formed by such toners is as shown in FIG. 3 and it is sprayed with a solvent 3 as indicated in FIG. 4, the toners are solved under the action of the solvent, and adhesively deposited on the surface of the substrate. If a second electrophotographic procedure is desired to be carried out on such a surface of the substrate by dispersing toner thereon, charging the toner, and exposing the toner to light to form an image, great difficulty has been experienced because of the existence of an insulating layer consisting of the already developed toner under the newly dispersed toner, at the region represented by 21 in FIG. 4 which is solved toner. The newly dispersed and charged toner thus cannot be discharged even if they are exposed to light rays. On the other hand, the newly dispersed toner in region 22 can be easily discharged. This means that the second image is seriously affected by the existence of the first image, and reproduction with high fidelity of the second image becomes almost impossible.

SUMMARY OF THE INVENTION Therefore, one object of the present invention is to provide an improved electrophotographic process whereby the above described drawbacks of the conventional electrophotography are substantially overcome.

According to the present invention, in an electrophotography process wherein a substantially uniformly distributed powder layer consisting of minute photoconductive toner which consists of cores made of an insulating thermoplastic resin provided with a photoconductive layer is formed on a substrate having a conductive surface, said toners being thereafter electrostatically charged, a first lightimage being focused on said toner layer, toner in regions strongly irradiated by light rays being removed by 'a suitable procedure, thus forming a first toner image, said toner image being thereafter fixed, said steps being development of the first image so that regions having toners remaining are made electrically conductive to a desired extent. That is, the invention is characterized in that a treatment to convert the already developed toner to a conductive state is carried out before subsequent deposition (1) to overcome the highly insulative nature of the already developed toner layer.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 through 3 are schematic sectional views for showing image forming processes employing photoconductive toners, which constitute fundamental processes in electrophotography.

FIGS. 4 and 5 are diagrams showing steps of electrophotography according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION As described before, the photoconductive toner employed in the present invention has a highly advantageous structure as disclosed in Japanese patent publication 12,385/ 1969. Such a toner is converted into a layer by means of a solvent or heat melting the core of the toner. The core of the toner, when it is made of glass or quartz, is developed (or deposited) on the substrate by spraying a resin solvent on the toner, and in this case an insulating layer is also formed on the substrate.

In order that the subsequent image forming steps can be exercised without being influenced by the previously formed image, it is necessary that the previously formed image portion has a surface resistance value of ohms per square-cm. or less, and the exposed portion of the substrate should also be adjusted below the above-mentioned value. If the substrate has sufficient electrical conductivity after the image has been developed, only the image need be rendered conductive. This may be done by electrostatic spraying of conductive particles charged opposite in polarity to that of the developed image on which charge remains.

The nature, principle, and utility of the present invention will be better understood from the following detailed description of the invention when read together with the accompanying drawing.

In FIG. 1, numeral 1 designates a substrate made of an electric conductor such as a metal, an insulator having a conductive layer on the surface, or a semiconductor. In any case, the substrate or surface layer of the substrate should have a surface resistance less than 10 ohms per square-cm. This can be realized by a thin layer of an electric charge preventing agent or a conductive resin. Many of such materials have an adhesive nature, but in this invention, since the photoconductive toner should not be adhered to the surface layer of the substrate 1, materials having a low adhesive nature are selected for this purpose. This requirement is also found in the hereinafter described treatment for obtaining electrical conductivity. Numeral 2 designates photoconductive toners having a construction as disclosed in the Japanese patent publication 12,385/1969. Typically, where fine particles of 10 to 100p. are sprayed on the surface of the substrate, a surface which does not show the affinity for the particles is preferred. In other words, the particles are easily removed by a weak physical force. Thus, the particles do not exhibit tackiness.

These toners are electrically charged before, simultaneously, or after they are dispersed on the surface of the substrate 1. The relation between the amount of the toner dispersed and their performance characteristics was disclosed at the International Conference for Electronic Photography held at Rochester in September 1968. The amount of toner dispersed is suitably 50-200 g./crn. and preferably 80 to 120 g./cm. (Toners having cores made of a transparent plastic resin are also described in Applied Optics, supplement 3, on Electrophotography (1969), pp. 124-128; H. Goto, K. Ohbuchi, S. Satamatsu,

4 l and H. Kato, The Image Forming Process of Photoconductive Toner.)

After exposure and development, the condition shown in FIG. 3 is obtained. Development can be achieved by blowing air or by rotating glass beads of a comparatively large diameter along the surface of the substrate. The fixing step shown in FIG. 4 may be achieved by spraying a solvent over or by applying heat to the remaining toners. Whenever the core material of the toners cannot be melted by heat or dissolved in a solvent (although complete melting or dissolution is not required), the fixing step is attained by spraying a solution of a binding synthetic resin thereon such as silicone resin, polyvinyl acetate, polymer of vinyl chloride and vinyl acetate, polyacrylate, polymethacrylate, etc.

The electrical conductivity treatment constituting the characteristic feature of the present invention may be carried out simultaneously with this fiXiIlg step, or as an independent step. In the former case, a solvent or solution containing an electrically conductive resin or charge preventing agent is employed in the fixing process for achieving fixation and rendering electrically conductive simultaneously.

Since many resins rendering a surface electrically conductive and charge preventing agents are soluble in a. polar solvent (such as water, methanol, and ethanol) and are inconveniently contained in ordinary fixing solution, the charge preventing agent or resin can be made into a separate solution and sprayed on the already fixed image.

When the fixing and rendering conductive are carried out simultaneously, a resin or agent for rendering conductive which is not easily dissolved in the fixing solution may be dispersed in the form of colloid, or reversely, the fixing resin solution may be dispersed in the form of colloid into the agent rendering conductivity.

As conductivity rendering agents, quaternary ammonium salt polymers (polyvinyl-benzyltrimethylammonium chloride, etc.), polyvinyl benzene sulfoxide, colloidal alumina, colloidal silica, etc. may be employed. Furthermore, quaternary ammonium salt type cation-activating agents or the like may be used for the same purpose. In addition, the techniques disclosed in US. Pat. 3,295,967, US. Pat. 3,228,770, or in Japanese Pat. 14,035/1969 may also be utilized. Since the polymer or boundary surface activating agent tends to exhibit an adhesive nature, colloidal alumina or colloidal silica may be dispersed therein to reduce adherence.

It can thus be seen that in the present process the selection of any one particular conductivity rendering agent or charge preventing agent is not overly critical. However, the material can be selected from a broad class, the above materials being only exemplary, so long as the conductivity and adhesivity requirements heretofore posed are met. It will be apparent that the ideal material has a non-adhesive nature for the toner later deposited and that as the adhesivity therefor increases, the excellent results obtainable in the present invention become lessened.

It is important that in the conductivity rendering treatment the color and quality of the image already formed are not damaged. The above described conductivity rendering agents form a transparent layer over the image, and for this reason, they can be used for obtaining multi-color images. From 1 g. to 10 g. per onesq. meter of conductivity rendering agent is applied onto the image surface, and this amount is not significantly changed even when the agent contains other high resistance components. The important criterion is that the surface resistance of the image area be less than 10 ohms per sq. centimeter.

The substrate thus treated permits the subsequent steps to be carried out regardless of the existence or nonexistence of the toner image on the substrate. FIG. 5 indicates a state wherein the electric conductive layer 3 is formed over the toner image.

* EX M Ev j I A mixture of the following, substances was'further ground andmixed in a ball mill.

Photoconductive zinc oxide (Sazex #2000 manufactured by Sakai Chemical Ind.) parts by wt-.. Silicone resin varnish FSR-107 50% weight content (made by Fuji KobunshiInd. Co.) parts Cyclohexane do 20 parts of the'thus ground and dispersed mixture was added to 70 parts of polymethyl-methacrylate resin granules of an average grain size 70 microns (and having 2.5 mm.- of absorption factor for a radiant rays of 3800 A. which is within the specific sensible wavelength range of zinc oxide), and was stirred to obtain a coated photoconductive powder in the dried condition.

The photoconductive toner thus obtained was dispersed on a conductive black paper (carbon containing paper having a surface conductivity of about 10- mho square at 50% RH) at 80 g./m. of the surface area, and the black paper thus carrying toner particles was electrically charged in a dark room through corona discharge to have a negative polarity of about 300 volts. After the black paper was exposed to an image light of about 1000 lux sec. an air stream was blown onto the paper so that a white image was obtained on the background of black color.

Over the thus formed image on the black paper, a fixing and conductivity rendering liquid made of parts of ECR-34 (Dow Chemical Co. aqueous solution containing 35% by weight polyvinyl-benzyl-trimethyl-ammonium chloride), 30 parts of Almina Sol 100 (Nissan Kagaku Co., an aqueous system colloid containing about 20% by weight colloidal almina), 20 parts of acetone, 50 parts of dimethylformamide, and 90 parts of methanol, was sprayed onto the element so that a 2 g./m. non-volatile layer was left on the image.

After the first image was thus completely fixed, a second image was formed over the first image employing a photoconductive toner prepared as follows.

The above substances were ground and kneaded in a ball mill for about 17 hours. 30 g. of the thus ground substances were added to a 100 g. of granular powder which was prepared by sieving an acrylic base MH-105 (made by Fujikura Kasei Co., having an absorption factor of 1.8 mm. for a radiant ray of 5500 A. wavelength within the specific sensitive wavelength range of cadmium yellow-orange) to have a minimum grain diameter of 35 microns and a maximum grain diameter of 104 microns. The mixture was further stirred and dried by the application of hot air wind and lightly ground in a mortar into a powder-like structure.

The thus obtained powder was dispersed over the first image at a rate of 0.008 g./cm. electrically charged to an initial potential of about 260 volts, exposed to light forming a second image employing a xenon lamp, developed employing compressed air suitably blown onto the surface, and the second image of yellow-orange was obtained without any influence from the first image. The second image was fixed employing dimethylformamide and an image having two colors was obtained.

For the purpose of comparison, the first image was simply fixed by dimethylformamide, dried, and then the second image formed as described above. In this case, the

yellow-orange toner was adhered over the first image, and no fidelity of reproduction was realized.

EXAMPLE 2 By the same procedure as described Example 1, a

first photoconductive toner prepared as follows was employed. An art paper having a surface conductivity of about 10 mho square at 70% RH was used as the substrate. g I Parts Acrylic base MH-l05 (by Fujikura Kasei Co.polymethacrylic-acid ethyl granular polymer, having a 60 to 70 K-value, grain size 60-100 meshes) 50 Fuchsine 0.1 Methylene chloride 450 Parts Photoconductive zinc oxide (Sazex #2000) 150 Polystyrene 15 Cyclohexane 250 The thus obtained core bodies were red when they ware fixed by means of a solvent. For the fixation, a mixed liquid at 1:1 weight ratio of dimethylformamide and methylene chloride was employed. For the'conductivity rendering treatment, a liquid prepared by mixing parts of a 10% water solution of polyvinylbenzene potassium sulfonate, 40 parts of a colloidal silica Water dispersion (containing 20% of non-volatile substances), and a trace of dodecylbenzene sodium sulfonate, was sprayed over the first image in an amount rendering a 3 g./ml. nonvolatile layer after being dried.

A second image was formed as described in Example 1. Equivalent results were obtained.

EXAMPLE 3 aqueous solution is 50 cp.) 10 Water emulsion of polybutylmethacryate (nonvolatile substance are 20%) 50 Water This treatment liquid was not repelled from the toner image of water repellent nature due to the existence of the anion activator contained in the emulsion.

Numerous modifications of the invention will become apparent to one of ordinary skill in the art upon reading the foregoing disclosure. During such a reading it will be evident that this invention provides a unique electrophotographic process for accomplishing the objects and advantages herein stated.

What is claimed is:

1. In an electrophotography process wherein a uniform powdery layer consisting of first photoconductive toner particles having a core body made of a transparent insulating substance and a photoconductive layer provided on the surface of the core body is formed on a substrate having a suitable electrical conductivity, said powdery layer being electrically charged, exposed to a first light image, the toner particles located in the nonimage region which is strongly irradiated by light being removed-from-the surface of the substrate,tand.the remaining toner in the image region being fixed to said substrate to prdiiide a first image where at least one additional layer of second photoconductive toner particles of a color diflerent than said first particles is formed uniformly on the'image and non-image regions of the same substrate, and the above steps for obtaining an image on the same substrate arerepeated, the improvement wherein, after the formation of one image but before the formation of a subsequent photoconductive toner layer, at least the toner image is rendered electrically conductive by'the application of a transparent, electrically conductive material showing substantially no adhesivity for subsequently applied toner so that a transparent, electricallyconductive layer is formed over at least the toner image, the substrate including the initial toner image thereby being provided with a surface resistance less than 10 ohms per square centimeter.

2. A process as in claim 1, wherein the conductivity treatment and the fixing are carried out simultaneously.

3..,A. processasrin claim *1, whereinsaid material for conductivity treatment comprises colloidal alumina.

References Cited i H UNITED STATES PATENTS 2,924,519 2/1960- Bertelsen 96 1.4 3,607,363 9/1971 Sadamatsu et a1. 96'-1.5 X 3,060,020 :10/1962 --Greig 96 1.2 3,157,546 11/19 4 Cover -96 .-1.4 X 3,672,988 6/1972 -Tamaietal. 96-1. 8 X j FOREIGN PATENTS 990,538 4/1965 Great Britain 96 1.4 1,082,914 9/1967 Great Britain 96-111 CHARLES E. VAN HORN, Primary Examiner U.S. C1.'X.R. 

